China announced Monday that its gross domestic product growth slowed to 7.5% in the three months ended in June. That’s healthy growth but way down from more than 12% six years ago. If China’s growth rate sticks at 7.5% or less this year, 2013 will be its weakest year since 1990 when China started to redraw the world’s economic map .

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While the changes in the Chinese economy are reverberating across the globe, Southeast Asia is particularly vulnerable as it has seen its trade with China skyrocket in the last 15 years. The growth has been powered not only by China’s expansion but also by a free-trade agreement between China and the Association of Southeast Asian Nations which lowered trade barriers in 2010.

Total trade between China and the 10 members of Asean has grown more than 30-fold in the last 15 years. That compares to the not-quite tripling of trade between Asean and the Japan or the U.S. over the same period. China is now Asean’s largest trading partner. Ten years ago, it was fourth-largest, well behind the United States, Japan and the European Union.

China’s until recently seemingly insatiable demand for commodities had been a big boost for Southeast Asia’s largest economy, Indonesia, which exports much of its natural resources, including coal, tin, rubber, cocoa and palm oil, to China.

As demand from China has slowed, commodity prices — notably coal and palm oil — have lost ground in the last year. Indonesia’s coal companies, rubber tappers and oil palm plantation workers are now worried that the good times may be ending.

Indonesian coal company PT Lumbung Energi & Metal says it has cut its expansion plans in half this year as demand from China and coal prices have tapered off. Around half of its exports go to China.

“We’re still monitoring the China situation to see whether they’re stabilizing or will they slow further,” Borneo chief executive Alexander Ramlie told the Wall Street Journal.

Palm oil plantation owner Nasir Sihotang, 55, says his income has dropped 30% in the last year as the price he can get for the palm fruit grown on his 10-hectare farm in eastern Sumatra has fallen. Demand from China has waned just as production capacity in the region has been increasing from all the new plantations that sprouted up when prices were higher.

“There’s the China factor because demand is slowing there and we are also now dealing with excess production,” he said. “We’ve have been forced to cut down on our household spending, including food. Some of my fellow farmers have even been forced to stop sending their children to school.”

Indonesia’s gross domestic product expansion in the first quarter was the lowest it has been in more than two years. Some economists expect Indonesia’s growth to dip below 6% for the first time since the 2008.

Singapore, a regional financial center where trade flows dwarf the small domestic economy, has also benefitted greatly from China’s economic boom. It has supplied significant inflows of Chinese cash from tourists and investors who frequent the city-state’s attractions and buy up its properties.

While Singapore hasn’t felt much pain so far from slowing economic growth in its third-largest trading partner, economists say a sustained slump could eventually hurt its electronics, pharmaceutical and oil product exporters .

Many Singaporean corporations, which have big footprints on the mainland, including as real-estate group CapitaLand Ltd. and warehouse developer Global Logistic Properties Ltd., could also be hit if the growth in Chinese demand wanes.

In Thailand, exports shrank 5.25% in May from a year earlier due to the contraction in global and Chinese demand. Rubber as well as computer parts which are exported to China have been among of the hardest-hit sectors.

Thai Rubber Latex Corporation, the world’s largest exporter of latex concentrate, said it has been feeling the slowdown in China since last year. It exports more than half of its output to China.

“After double-digit growth rates in recent years, we are prepared to see sales flatten out or even fall in 2013,” said Paitoon Wongsasutthikul, an advisor to the company’s board of directors.

About Southeast Asia Real Time

Indonesia Real Time provides analysis and insight into the region, which includes Singapore, Thailand, Indonesia, Vietnam, Malaysia, the Philippines, Myanmar, Cambodia, Laos and Brunei. Contact the editors at SEAsia@wsj.com.

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